When your email goes to spam, the cause is almost never one single thing. Mailbox providers combine hundreds of signals into a decision about each message. The good news is that most failures come from a short list of causes you can check in about an hour. Work through them in this order, because the early ones are the cheapest to fix and the most common.
1. Missing or broken authentication
If a message is not authenticated with SPF, DKIM and DMARC, providers treat it as untrustworthy. Common faults are two SPF records, a DKIM selector that does not match, or no DMARC record at all. Send a test message to a Gmail address and read the results, as shown in how to check SPF, DKIM and DMARC.
2. A brand-new domain or sending address
New domains have no history. A sudden burst of mail from a domain registered last week looks just like a spam operation. Start with low volume to engaged recipients and increase slowly. See how to warm up a new domain.
3. Poor sender reputation
Providers track how recipients react to your mail: opens, replies, deletes without reading, spam complaints. A history of complaints or bounces drags every future message down. Read sender reputation explained.
4. Sending from a shared or tainted IP
If your mail goes out through a server that other, careless senders also use, their behaviour affects you. Check whether your sending IP appears on public blocklists. A reputable provider keeps abusive customers off its infrastructure.
5. Mismatched From, Reply-To and links
A From address at one domain, a tracking link at another, and an image hosted at a third looks like phishing. Keep the visible domain, the link domain and the signing domain consistent wherever you can. Never use a visitor's address as the From address for a contact form; send from your own domain and put the visitor in Reply-To.
6. Spammy content and structure
Filters score the content: all-capital subject lines, many exclamation marks, "free money" language, a message that is one big image, a URL shortener, or a huge attachment. The old advice about "spam trigger words" is overstated, since context matters more than any single word, but the overall impression counts. See what spam filters actually look at.
7. HTML-only messages
Always send a plain-text part alongside the HTML. A message with only HTML, especially one with broken markup, scores worse than a clean multipart message.
8. Bad list quality
Sending to addresses that bounce, to old lists, or to bought lists produces hard bounces and spam traps. A bounce rate above a couple of percent is a warning sign. Remove addresses that bounce permanently and never buy a list. Learn the codes in email bounces and SMTP error codes.
9. No easy way to unsubscribe
For anything marketing-like, a visible unsubscribe link and the one-click header make recipients unsubscribe instead of hitting "report spam". Complaints are far more damaging than unsubscribes. See List-Unsubscribe and one-click unsubscribe.
10. Sudden volume spikes
Sending 50 messages a day for months and then 5,000 in an hour triggers rate-based defences. Ramp gradually and spread large sends over time.
11. Mixing different kinds of mail on one domain
Password resets and receipts have high engagement. Newsletters have lower engagement and more complaints. If both share one sending identity, the newsletter's problems can hurt the important messages. Use separate subdomains, for example app.yourdomain.com for transactional mail and news.yourdomain.com for newsletters.
12. Technical hygiene problems
A sending server needs a valid reverse DNS (PTR) record matching its hostname, TLS support, and correct HELO identification. If you use a hosted provider these are handled for you. If you run your own server they are on your checklist.
A quick triage table
| Symptom | Start with |
|---|---|
| Everything goes to spam | Authentication, then blocklists |
| Only Gmail goes to spam | Gmail rules in Gmail and Yahoo requirements |
| New domain, first sends | Warm-up and low volume |
| Worked last month, fails now | Reputation: complaints, bounces, new list |
| Contact-form notifications go to spam | From address mismatch, no Reply-To pattern |
What you cannot control
No sender can guarantee inbox placement. Final decisions belong to the mailbox provider, and they depend on the recipient too: a message from someone in a contact list is treated differently from a stranger. What you can do is remove every avoidable negative signal and build a clean record over time. A practical pre-launch list is in our email deliverability checklist.
How Mailions helps with the basics
Mailions handles the technical parts for you: a dedicated DKIM key per domain, the SPF, DKIM and DMARC records to publish, and a live DNS check that flags a missing or duplicated record. Sending logs show the relay result of each message. Authentication supports good placement but does not guarantee it; reputation, content and recipient behaviour do the rest.